In recent years, display panels such as large flat panel displays and small lightweight displays are increasingly slimmed down and lightened through, for example, by slimming down of display components and by direct affixation of optical members. As a result, maintenance of impact resistance of these display panels is an important issue. In addition, as associated demands adhesive materials for affixing optical members to display panels are required to have removability from the display panels (reworkability). Conventionally, when an affixation failure occurs during production, the failed optical member is removed from the display panel, and affixed to the panel again. However, panels having a low strength can be broken during removal of the member, which results in the deterioration of the yield of expensive panels. The yield can be maintained or can be improved by improving the removability of the adhesive material.
In order to solve the above-described problems, for example, Japanese Patent Application Laid-Open No. 2002-23649 discloses a shock absorption laminate having a complicated multilayer structure composed of a shatterproof layer, two anti-cracking layers, and a transparent adhesive layer. However, from the viewpoint of practicality, the quality and cost of the laminate are not satisfactory.
Japanese Patent Application Laid-Open No. 2005-249854 discloses a shock absorption layer composed of a polyurethane resin, specifically an urethane resin prepared by radical polymerization of an oligomer having one to three (meth)acryl groups at the ends. Obviously an urethane resin is a material whose properties such as hardness, resiliency, and stickiness after curing can be widely varied by, for example, changing the combination of isocyanate and polyol components. However, Japanese Patent Application Laid-Open No. 2005-249854 does not specify the urethane resin, but only gives general explanation of isocyanates.
Japanese Patent Application Laid-Open No. 2001-316447 discloses a soft composition having stickiness with rubber hardness of 30 or less, the composition being prepared from a polyol having 2.4 to 3.0 functional groups and a molecular weight of 3,000 to 6,000, and a secondary or tertiary higher monoalcohol as the tackifier. In Japanese Patent Application Laid-Open No. 2001-316447, curing reaction is carried out through urethane reaction, and thus requires a long process including reaction at 100° C. for 2 hours, aging for 7 days and the like. Therefore, the method is suitable for cast molding, but unsuitable for other production methods more efficient than cast molding, for example, a method for producing a thick film sheet, wherein a composition is continuously applied onto a base film without including bubbles.
Japanese Patent Application Laid-Open No. 2004-359808 discloses a transparent gel adhesive and a sheet including the same, the transparent gel adhesive being composed of a three dimensionally crosslinked polymer swelled with a liquid containing a plasticizing agent and inorganic particles. In Japanese Patent Application Laid-Open No. 2004-359808, breeding tends to occur due to the use of a plasticizing agent, which may result in the contamination or corrosion of the adherend.
The above-described disclosures may partially improve the impact resistance of a display panel. However, it is not found adhesive material having impact resistance, combining adequate adhesion with removability, and providing mass productivity, transparency, and non-polluting properties.